As can be seen in FIG. 1, a semiconductor wafer 14 generally has a highly polished flat mirror like top surface 4 which is made from silicon or some other semiconductor material. The wafer generally has a diameter 2 of between four and twelve inches. Various marking indicia are placed upon a top surface of the wafer to provide pertinent information concerning the identity, type or content of the wafer. The marking indicia is typically in the form of individual dots which comprise characters and/or symbols. Accurate reading of that information, during the wafer manufacturing process, is critical for further processing of the wafer.
Currently, an alphanumeric string or a bar code 6 is formed on a top surface 4 of the wafer 14 to identify the wafer 14. As the manufacturing cost associated with producing a wafer is very high, manufacturers wish to utilize substantially the entire surface area of the wafer to maximize production of desired electrical components. Accordingly, there is an increasing demand to place smaller and more compact indicia, e.g. a matrix code 8, on the surface of the wafer 14 which can contain as much as ten times more relevant information than a typical bar code or an alphanumeric string 6. However, due to the compact size and optical characteristics of the matrix code 8, it is much more difficult to read accurately by currently available vision systems. In an attempt to partially overcome some of the reading difficulty, the matrix code 8 contains some redundancy so that even if a minor error(s) is made in reading the matrix code 8, all of the relevant information can still be accurately deciphered by the vision system.
The marking placed on a wafer surface generally takes one of two forms, i.e. either a "hard mark" or a "soft mark". A hard mark is generally formed by a high powered laser marking process which etches an array of microscopic pits into a desired wafer surface to form a desired pattern. Upon formation of a hard mark, the material is ablated from the surface to yield a microscopic, steep edged pit. One of the drawbacks associated with a hard mark is the generation of dust and other process contaminants, upon creating the array of microscopic pits. The industry as a whole has been moving toward utilization of soft marks 10 which, as can be seen in FIG. 2, typically consist of an array of shallow, concave depressions formed in the top surface 12 of the flat semiconductor wafer 14. Soft marks are typically formed by a low powered laser marking process in which the surface is only slightly deformed to form the mark. The concave mark or depression is on the order of about 75 .mu.m in diameter and 1 .mu.m-3 .mu.m in depth. Normally, the formation of a shallow, concave depression or soft mark results in displacement of the adjacent wafer surface material so as to form a slightly raised perimeter edge 16 which circumscribes the shallow, concave depression or soft mark 10. Due to their shallow depth, soft marks are substantially more difficult to image than hard marks.
The central surface area 18 of each concave depression making up the soft mark may retain a high degree of specularity, after the laser marking process, although its specularity is somewhat less than that of the surrounding undisturbed flat surface area 19 of the wafer 14. The raised perimeter edge 16, which circumscribes each formed concave depression or soft mark 10, has a tendency to scatter the supplied light in a plurality of different directions and thus is an imperfect reflector of any supplied light. The indicia and the adjacent surrounding wafer surface area therefore differ both in their geometry and their surface specularity, with each indicia forming an extremely shallow concave, slightly "dirty" mirror in the surface of a flat clean mirror. The slight physical differences, between the soft marks and the adjacent surface area, are the only features that distinguish the marking indicia from the surrounding surface area 19 of the wafer 14.
Because any process which alters the geometry of the wafer surface has the potential to result in contamination of the semiconductor wafer manufacturing process, it is desired that the indicia marking process for the semiconductor wafer results in a minimum alteration of the wafer surface. As the power and time duration of the laser beam utilized to create marking indicia increases, the geometry of the formed indicia differs more greatly than the geometry of the surrounding flat surface, thereby making them easier to image as well as more prone to contamination during the manufacturing process. Conversely, as the power and time duration of the marking laser beam are reduced in order to minimize the potential for altering the wafer surface and contaminating the manufacturing process, the geometry of the formed indicia differs only slightly from the geometry of the surrounding flat surface and the marks become correspondingly more difficult to image. It is to be noted that laser etched indicia can take a number of forms, depending upon the geometry, the time domain and the energy profile or other attributes of the laser beam used to create the indicia.
The marking indicia is typically imprinted on the semiconductor wafer very near an outer edge (FIG. 1). After laser marking, the semiconductor wafer goes through the semiconductor manufacturing process where a number of thin layers of different materials are deposited on the wafer's top surface 4 resulting in a complex circuitry of individual semiconductor dies and a change in the surface density of the wafer. While the various surface deposition layers are extremely uniform across the interior of the wafer, where the individual dies are formed, it is characteristic of the various depositions to have non-uniform boundaries near the edge of the wafer where the marking indicia are located. The various wafers can have a smooth or a highly variable broken edge and these variable surface features can be superimposed on or over the marking indicia, greatly increasing the difficulty in properly imaging them to distinguish them from the surrounding surface. The roughly formed deposition layer boundaries, described above, typically has a surface gradient or slope which extend in a direction perpendicular to an adjacent edge of the wafer, i.e. the boundary between the two layers typically extends parallel to the edge but the gradient or slope between the two layers typically extends in an opposite direction which is perpendicular to the edge.
A further problem associated with accurate imaging of a matrix code containing soft marks, is that one or more coatings 17 may be applied to the top surface 12 of the wafer 14 and such coating(s) 17 may partially or completely overlay the hard marks or the soft marks 10 comprising the matrix code 8. Such coatings 17 can be either clear or opaque but are typically shiny and cause additional problems in accurately reading the matrix code 8.
The entire marking indicia on a semiconductor wafer typically ranges in size from about 2 millimeters by 2 millimeters up to about 42 millimeters by 42 millimeters. A single wafer may exhibit both sizes of marks. An imaging system with a field-of-view large enough to image the largest mark, typically has a resolution which is too course to image the smallest mark. Therefore, a comprehensive imaging solution must include the ability to provide different fields-of-view so that all types of marking indicia can be accurately imaged.
As can be seen in FIG. 1A, an alphanumeric string 6 comprises a plurality of tiny pits created in a top surface of the wafer 14 to generate a code consisting the letter "A" and the numbers "170", i.e. "A170". FIG. 1B shows a portion of a matrix code 8 which is also formed from a plurality of tiny pits in a top surface 4 of the wafer 14 to generate a desire code.
One prior art method used to illuminate surface marking indicia in a wafer, with an optical axis lying normal to the surface being imaged, is to provide illumination almost adjacent to, but not coincident, with the optical axis. As the radius of curvature of the marking indicia increases, due to lower power laser markings, the reflection geometry of the marking indicia requires that the angle between the optical axis and the nearly coaxial light supply source be very small, e.g. on the order of one degree or so. For example, marking indicia with a surface slope of one degree from horizontal will not reflect light from an off-axis light source along the optical axis until the light source is within two degrees of the optical axis.
Because the camera is looking at a flat mirror surface, the field-of-view consists of a reflection of a solid angle around the optical axis, which includes the camera aperture, plus an adjoining area whose dimensions can be readily calculated by known techniques. Since the flat mirror surface reflects an area surrounding the optical axis which is larger than the camera lens aperture, the light source is brought very close to the optical axis. The light source, in an attempt to minimize the off-axis angle for the purpose of imaging surface marking indicia, will at some point appear reflected in the periphery of the field-of-view. As the light source is brought close enough to the optical axis so that the shallow angled surfaces of the individual marking indicia are able to reflect light back to the camera lens, thereby appearing bright on a dark background, the reflection of the light source on the adjacent flat mirror can appear so close to the marking indicia themselves that their images may overlap. In addition, as the specularity of the flat mirror surface, surrounding the marking indicia, is significantly greater than the specularity of the laser-etch reflective central curved surface of the marking indicia, the flat surface reflection of the illumination source appears much brighter in the camera's field-of-view than the individual marking indicia.
The apparent brightness of the marking indicia may be increased by increasing the intensity of the light source or by using a camera lens with a wider aperture which is capable of capturing more light. As the intensity of the light source is increased to increase the brightness of the marking indicia, a much brighter adjacent direct reflection of the light source may "bloom", due to photo-electric properties of the camera imaging array, and appear sufficiently large so as to impinge on the less bright marking indicia. Additionally, a larger aperture camera lens will reduce the depth of focus and also may cause the reflected image of the light source to be overly bright and thus appear more out-of-focus and possibly impinge on the less bright marking indicia. Therefore, the prior art methods which simply seek to bring a diffuse light source close to the optical axis are limited, by the above noted effects, in how close to the optical axis the off-axis light source can be brought. These effects set a corresponding lower limit of the angle of the surfaces of the marking indicia which may be imaged with such techniques, and hence a lower limit on the degree of disturbance of the wafer's surface which must be tolerated by the manufacturing process.